The mouse gene trap strategy is an insertional mutagenesis involving an exogenous DNA, termed the trap vector, as a mutagen that produces a mutation in the mouse genome and a sequence tag to facilitate the isolation of the mutated genes. The trap vector consists of a reporter gene whose expression mimics that of the endogenous genes mutated and a selection marker that sorts cells bearing the inserted vector. Gene trap is a powerful method for identifying genes important in biological phenomena. Moreover, the method produces mutant organisms whose phenotypes provide invaluable information about the biological functions of the genes responsible for these phenotypes. Indeed, a number of genes essential for mouse embryogenesis have been identified by the gene trap method. Here, we describe the principle, results, and perspectives for applications of gene trap approach to the study of cell differentiation and lineage commitment.